A typical mining shovel includes a turntable mounted on a crawler truck, and supporting an A-frame and a cab. A boom extending from the turntable has an upper end supported by the A-frame. The boom pivotally supports a dipper handle which pivots in a vertical plane. A dipper fixed to a distal end of the dipper handle is raised and lowered by a hoist cable which extends over a sheave at the top of the boom and down to a padlock on the dipper. The hoist cable provides for the vertical, raising and lowering, movement of the dipper. A crowd mechanism extends and retracts the dipper handle to provide the horizontal component, or crowd, of the dipper's movement.
Many different crowd mechanisms have been developed over the years. Rack and pinion crowd mechanisms include a rack fixed to the dipper handle which engages a rotatably driven pinion, or gear, mounted in the boom. Rope crowd mechanisms include metal ropes that are wound and unwound from a crowd drum to extend and retract the dipper handle. Hydraulic crowd mechanisms, such as disclosed in U.S. Pat. No. 3,425,574, which utilizes a large double-acting hydraulic actuator are also known in the art. All of these mechanisms have advantages and disadvantages.
Hydraulic crowd mechanisms can use round tubular handles that are free to rotate while the rack and pinion mechanism must remain rotatably fixed. Moreover, hydraulic crowd mechanisms are not prone to broken rack teeth or ropes resulting from excessive force, such as in the gear and rack crowd mechanism or the rope crowd mechanism. Unfortunately, in the prior art, the volume of hydraulic fluid necessary to control the crowd of a mining shovel dipper handle prevents a quick response to operator inputs compared to the other crowd mechanisms because of the mass of the spools in control valves used to control the flow of the hydraulic fluid. Accordingly, a need exists for a hydraulic crowd mechanism which responds quickly to operator inputs compared to the prior art.